It is good mental model because whoever is producing the software is human.

It does not matter if there is zero cost to copying the software.

What matters is that somebody put an effort into doing something and now they are seeing lots of people using it for free. It is in human nature to try to benefit from it and most people have a mental model that they deserve to be compensated for doing something beneficial to another.

Most people are not seeing incremental cost of adding another user (effectively zero or non zero), what they are seeing is total cost of producing the software divided by the number of users. Then they see that some users are paying and some users are not.

Being copyable for free is one thing but there is a cost to create ... Developer brain power is needed for that ...